1. Field of the Invention
The present invention relates to a process for treating chemical pulp for paper manufacturing. More particularly, the present invention relates to an improvement in the treatment for delignifying and bleaching chemical pulp.
2. Description of the Related Arts
Bleaching of chemical pulp for paper manufacturing is conducted by bleaching treatments in many stages. For the multi-stage bleaching, bleaching chemicals containing chlorine have heretofore been used. More specifically, the bleaching is conducted by combined use of chlorine (C), a hypochlorite (H), and chlorine dioxide (D), such as the combined use in a sequence of C--E--H--D, or C/D--E--H--E--D (C/D represents a stage of bleaching by the simultaneous use of chlorine and chlorine dioxide, and E represents a stage of extraction with an alkali).
However, toxic organic chlorine compounds detrimental to the environment are formed from the bleaching chemicals containing chlorine as by-products during the bleaching, and waste water containing the organic chlorine compounds discharged from the bleaching process causes an environmental problem. The organic chlorine compounds are generally analyzed and evaluated in accordance with the AOX method, such as METHOD No. 9020 of the United States Environmental Protection Agency.
For decreasing or preventing formation of organic chlorine compounds as by-products, it is most effective that the used amounts of chlorine chemicals are decreased or chlorine chemicals are not used at all. It is particularly effective that elementary chlorine is not used in the first stage. The pulp which is produced by this process is called an ECF (elementary chlorine free) pulp.
As the process for producing an ECF pulp from cooked pulp without bleaching with chlorine in the first stage, processes in which cooked chemical pulp is treated with an acid, and then the treated chemical pulp is bleached with hydrogen peroxide in an alkaline medium are disclosed in Japanese Patent Application Laid-Open No. Showa 51(1976)-102103 and Japanese Patent Application Laid-Open No. Showa 56(1981)-85489. A process in which chemical pulp bleached with oxygen in advance is treated with a chelating agent, and the treated chemical pulp is bleached with hydrogen peroxide or with a combination of hydrogen peroxide and oxygen in an alkaline medium is disclosed in Japanese Patent Application Laid-Open No. Heisei 3(1991)-27191. In the specifications of these applications, it is described that metals which cause waste in hydrogen peroxide by decomposition in the next stage of bleaching with hydrogen peroxide are removed by pre-treatment of chemical pulp with an acid or a chelating agent, and delignification and bleaching in the stage using hydrogen peroxide can more efficiently be conducted. However, these processes have a drawback in that the processes are inferior in delignification although sufficient bleaching can be achieved by the processes. Therefore, another drawback arises in that the load in the bleaching in later stages is increased, and cost for bleaching and cost for treatment of waste water are increased.
The bleaching with hydrogen peroxide is inferior in delignification. As a process for solving this problem, processes using salts of molybdic acid as an activation catalyst in an acidic medium for activation of hydrogen peroxide are described in Journal of Pulp and Paper Science, Volume 18, No. 3, Pages 108 to 114 (1992) and in Journal of Japanese Association of Paper and Pulp Engineering, Volume 49, No. 3, Pages 88 to 92 (1995). However, these processes have a drawback in that increase in brightness is small although delignification by these processes is superior to that by the conventional process in which the treatment is conducted in an alkaline medium. These processes have another drawback in that cost of bleaching is increased because expensive hydrogen peroxide is used in a larger amount than that in the conventional process using chlorine.
Japanese Patent Publication Heisei 2(1990)-221482, Japanese Patent Publication Heisei 4(1992)-245988 and Japanese Patent Publication Heisei 6(1994)-207390 disclose that hemicellulase or xylanase is used in a process in which chemical pulp is further delignified after the chemical pulp has been bleached with oxygen to decrease the amount of bleaching agent containing chlorine used in later stages. However, this process has drawbacks in that viscosity of pulp is decreased by enzyme treatment, that yield of pulp is decreased, and that cost of bleaching is increased because the enzymes are very expensive.
Japanese Patent Publication Heisei 3(1991)-40888, Japanese Patent Publication Heisei 5(1993)-163691, and Japanese Patent Publication Heisei 5(1993)-302285 disclose processes in which the amount of a bleaching agent containing chlorine used in later stages is decreased by using ozone. However, these processes have drawbacks in that viscosity of pulp and yield of pulp are decreased by ozone although brightness is sufficiently increased, and that cost of bleaching is increased because ozone is very expensive.
As described in the above, the processes using hydrogen peroxide, the processes using an enzyme, and the processes using ozone have been proposed in order to avoid the use of chlorine and to decrease the used amount of bleaching agent containing chlorine. However, all these processes have the above drawbacks, and none of these agents are used as the main agent for the ECF bleaching.
In the United States and Europe, a process using chlorine dioxide in place of chlorine in the first stage is mainly conducted. This process has an advantage that the conversion into the ECF bleaching can be achieved simply by using chlorine dioxide which is a conventional bleaching agent in place of chlorine.
However, in order to convert the bleaching with chlorine in the first stage into the bleaching with chlorine dioxide, the capacity of an apparatus for generation of chlorine dioxide must be increased to 3 to 5 times as large as the capacity required for the conventional process. Thus, a drawback arises in that investment cost is increased. Moreover, when the bleaching in the first stage is conducted by using chlorine dioxide, decrease in the amount of discharged AOX is limited, and when further decrease in the amount of discharged AOX is required, it is impossible that the requirement is satisfied by this process.